Capacitors are extensively used for storing an electrical charge in filters, analog-to-digital converters, memory devices, control applications, and many other types of integrated circuits. Capacitors, which generally include conductive plates separated by an insulator, have a capacitance contingent upon a number of parameters, such as plate area, the spacing between the conductive plates, and the dielectric constant of the insulator. A common capacitor construction is a vertical parallel plate capacitor (VPP), which includes a stack of conductive plates with adjacent plates in the stack separated by an insulator.
VPP capacitors may be fabricated when the stacked metallization layers of a multi-level interconnect structure are formed by back end of line (BEOL) processing. Although copper metallurgy is frequently used in lower metallization layers of the interconnect structure to increase signal propagation speed, aluminum metallurgy is preferred in upper metallization layers to promote solder and wire bonding. Consequently, the stacked plates of VPP capacitors may have either an aluminum metallurgy or a copper metallurgy correlated with the specific metallization layer in the interconnect structure. Virtue of the BEOL processing, each copper plate is clad along its sidewalls and bottom surface by a barrier layer containing one or more refractory metals. In contrast, each aluminum plate is clad on only its top and bottom surfaces by a barrier layer containing one or more refractory metals.
Circuitry elements in an integrated circuit, such as VPP capacitors, are susceptible to damage from excessively high voltages or currents generated by electrostatic discharge (ESD) events. ESD events may be caused by contact with the human body, by machinery such as manufacturing or test equipment, or in electrically active environments, as may be experiences in many consumer applications. In particular, the sudden and momentary discharge of an ESD event between adjacent aluminum plates or adjacent copper plates in a VPP capacitor can cause damage.
Under ESD testing and during ESD events in an operating device, aluminum plates have been observed to be more prone to damage and failure than copper plates. Generally, ESD-promoted failure may occur by crack initiation and propagation in the dielectric material bordering the conductive plate followed by melting and flow of the aluminum or copper from the plate into the crack. Aluminum plates, which are confined by refractory metal cladding on the top and bottom surfaces, fail by a lateral cracking mechanism, which promotes shorting of adjacent plates in a metallization layer. In contrast, copper plates are confined by refractory metal cladding on the sidewalls and bottom surface and, consequently, fail in a vertical direction. Hence, copper plates have an intrinsically higher resistance to ESD-promoted failure.
A methodology is needed for designing vertical parallel plate capacitors that exhibit increased resistance to ESD-promoted failures.